Certain cutting assemblies used for the removal of material may include a tool holder that contains a pocket, a shim and a cutting insert. Typically, the shim is affixed within the pocket such as, for example, by a shim screw or a threaded pin. The cutting insert typically rests upon the upper surface of the shim and is affixed (or firmly secured) thereto by a clamp member. There may be a mechanical chip breaker between the clamp and the cutting insert.
Certain material removal applications, e.g., a turning application, use a coated or an uncoated polycrystalline cubic boron nitride (PCBN) cutting insert. While current PCBN cutting inserts (coated and uncoated) perform adequately, a material removal application that uses such a cutting insert generates a great amount of heat. This heat generally passes into the shim upon which the cutting insert rests so that the shim becomes hot. In some situations, especially when the cutting length is particularly long, for example, one or more hours in a cut such as may be encountered with machining a mill roll such as a steel mill roll, this heat passes into the pocket region of the tool holder and causes the pocket to become hot resulting in oxidation of the toolholder assembly including the shim.
In the past, the shim has been made from a carbide-based material (e.g., cemented (cobalt) tungsten carbide). When the shim received the heat from the material removal application and thereby became hot (e.g., a temperature greater than about 400 degrees Centigrade), it oxidized wherein the oxidation was greatest in those portions defined by the surfaces exposed to the air. Because the oxidized portions of the shim (i.e., the oxidized tungsten carbide-cobalt material) had a lower density than the non-oxidized material, those portions of the shim that were exposed to the air increased in size. Such an increase in size caused a raised ridge in the region of the cutting edge in the area of the cut. Upon the indexing of the cutting insert, such a ridge resulted in the misalignment of the cutting insert with respect to the shim. This misalignment caused the cutting insert to chip under certain circumstances.
In addition, the oxidized tungsten carbide-cobalt material is itself very brittle. The oxidized tungsten carbide-cobalt material also has poor adhesion to the non-oxidized tungsten carbide-cobalt material. Upon continued use of the cutting assembly, the oxidized tungsten carbide-cobalt material falls away from the shim which leaves large pits along the edge of the shim where the rate of oxidation is the greatest and in areas beneath the PCBN cutting insert that oxidize more slowly. The result is the existence of pits and ridges in the shim that cause unstable seating of the cutting insert.
In a situation in which the pocket region of the tool holder would receive heat and reach a temperature in excess of about 400 degrees Centigrade, the region of the tool holder that defines the pocket would experience oxidation. Since the tool holder is typically made from steel, and in some cases it may be made from tungsten carbide, the oxidation would result in an increase in the size of the surfaces that define the pocket since the oxidized material has a lower density than the non-oxidized material. Under certain circumstances, such a change in the dimension of the pocket would result in the misalignment of the cutting insert upon indexing. Such misalignment could result in chipping of the cutting insert.
It would thus be desirable to provide a cutting assembly that includes a shim that maintains its dimensional integrity during the material removal operation. As a result, upon indexing of the cutting insert, alignment and support would be maintained between the cutting insert and the shim so as to minimize the potential for the chipping of the cutting insert. Further, alignment and support would be maintained so as to minimize the potential for loss of wear resistance because of an improper attack angle in the material removal operation even when the PCBN cutting insert does not chip.
It would also be desirable to provide a cutting assembly that includes a tool holder wherein the region of the tool holder adjacent to the pocket exhibits oxidation resistance so as to maintain its dimensional integrity. As a result, upon indexing of the cutting insert, alignment and support would be maintained between the cutting insert and the shim so as to minimize the potential for the chipping of the cutting insert. Further, alignment and support would be maintained so as to minimize the potential for loss of wear resistance because of an improper attack angle in the material removal operation even when the PCBN cutting insert does not chip.